CN109531544B - Two-degree-of-freedom parallel robot with space branched chain structure - Google Patents

Abstract

The invention discloses a two-freedom parallel robot with a space branched chain structure, which comprises: the device comprises a fixed platform, a movable platform, two driving devices and a sub-branched chain. Each driving device comprises an active arm and a driving unit, and the two active arms are positioned in the same reference plane. At least one driving arm is hinged with a tail end support, and the tail end support is connected with the fixed platform through a reinforcing rod. Each of the sub-branches includes: two axostylus axostyles and two chain bars, one of two axostylus axostyles is established on the initiative arm or on terminal support, and another is established on moving the platform, and two chain bars and two axostylus axostyles form the parallelogram. Two sub-branched chains are connected to one end support, the two sub-branched chains on the same end support are symmetrical relative to a reference plane, and the distance between the two sub-branched chains on the end support is larger than that on the movable platform. The movable platform of the robot can realize two degrees of freedom of movement, and the robot has the characteristics of high rigidity of the whole machine, high control precision, large working space and the like.

Description

Two-degree-of-freedom parallel robot with spatial branching structure

technical field

The invention relates to the field of parallel robots, in particular to a two-degree-of-freedom parallel robot with a spatial branch chain structure.

Background technique

With the development of the industrial level, the application requirements of robots and machine tools continue to increase, such as the rapid packaging, sorting and handling of light materials in the plastics industry, electronics industry, pharmaceutical industry and food industry; such as personalized small plane CNC operations cutting, printing and engraving, etc.; such as metal cutting, material welding and surface spraying in the field of manufacturing. The analysis shows that for the above two-dimensional point-to-point and arbitrary curve motion, only two degrees of freedom of plane movement can be realized.

The traditional plane two-degree-of-freedom mechanism is mostly realized in series, and there are problems such as error accumulation and large mass of moving parts. Different from the series mechanism, the parallel mechanism is a closed-loop structure composed of two or more kinematic branches. It has the advantages of compact structure, high rigidity and large bearing capacity, and has become one of the ideal choices for the design of robots and machine tools. However, while the parallel mechanism in the related art realizes two degrees of freedom in the plane, there are still some problems that need to be solved, such as the kinematics and structural complexity caused by redundant and passively constrained branch chains. In a parallel robot disclosed in the prior art, it is proposed to use parallel moving members on the active arm to improve the kinematic performance, but this technical solution still needs to be improved.

SUMMARY OF THE INVENTION

The present invention aims to solve one of the technical problems in the related art at least to a certain extent. To this end, the present invention proposes a two-degree-of-freedom parallel robot with a spatial branch chain structure. The two-degree-of-freedom parallel robot has the characteristics of compact structure, easy assembly, high control precision, and large working space. High-speed and high-acceleration characteristics can be achieved.

A two-degree-of-freedom parallel robot with a spatial branch chain structure according to an embodiment of the present invention includes: a fixed platform; two driving devices, each of which includes a driving arm and a driving unit for driving the driving arm to rotate , the drive unit is arranged on the fixed platform, and the two active arms are located in the same reference plane; the moving platform has a main plane that is always perpendicular to the reference plane; the sub-branch, each The sub-branch chains are connected between the active arm and the moving platform, and each sub-branch chain includes: two equal-length shaft rods and two equal-length chain rods, two said shafts. The rods are arranged in parallel, one of the two shaft rods is set on the driving arm, and the other is set on the moving platform, and the two ends of the two chain rods are respectively connected with the two shaft rods to form parallel Quadrilateral, each of the shaft rods and each of the chain rods are spherically hinged; wherein, at least one of the active arms is hinged with an end bracket, and the end bracket is connected to the fixed platform through a reinforcing rod, so The fixed platform, the active arm, the end bracket and the reinforcing rod form a parallelogram; and when the active arm is directly connected with the sub-branch, one of the sub-chains is connected to the active arm branch; when the main arm is provided with the end bracket, the sub-branches are connected to the end bracket to indirectly connect the main arm, and two of the end brackets are connected to one of the end brackets. Sub-branch, the two sub-branches connected on the same end support are symmetrical with respect to the reference plane; in addition, in the two sub-branches connected on the same end support, connected in The two shaft rods on the end bracket form an included angle greater than 0 degrees and less than 180 degrees, and the two shaft rods connected to the moving platform form an included angle greater than 0 degrees and less than 180 degrees, and are connected The distance between the two shaft rods on the end support is greater than the distance between the two shaft rods connected on the moving platform; the driving device, the end support and the connection on the same The two sub-branches on the end support together form a space branch of a two-degree-of-freedom parallel robot with a space branch structure.

According to the two-degree-of-freedom parallel robot with the spatial branch chain structure according to the embodiment of the present invention, through the arrangement on both sides of the end bracket, that is, the mutual influence of the active arm-strengthened rod combined rod and the double-branched combined rod, the entire parallel robot is made High stiffness, high control precision, easy assembly, performance consistency and retention are all outstanding.

In some embodiments, in each of the sub-chains, at least one anti-torsion rod is connected between the two chain rods, and the anti-torsion rod and the shaft rod are equal in length and parallel to each other.

Specifically, each of the chain rods is provided with a sliding hole extending along the length direction thereof, and the rotating shaft of the anti-torsion rod passes through the sliding hole.

In some embodiments, the moving platform includes a center plate and a plurality of pairs of ear plates, each pair of the ear plates is connected to the outer periphery of the center plate, and each pair of the ear plates includes two spaced apart ear plates provided with ear holes. ear plates, each of the shaft rods is connected to the ear holes of a pair of the ear plates.

In some embodiments, the driving arm is provided with a weight reduction hole.

In some embodiments, the end bracket comprises: a support plate, on which the active arm is hinged; a pair of upper extension ears, a pair of the upper extension ears are provided on one side of the support plate, The reinforcing rod is hinged between a pair of the upper extension ears; two pairs of lower extension ears, the two pairs of lower extension ears are connected to the other side of the support plate, and each pair of the lower extension ears is connected to one the shaft of the sub-branch.

Additional aspects and advantages of the present invention will be set forth, in part, from the following description, and in part will be apparent from the following description, or may be learned by practice of the invention.

Description of drawings

The above and/or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of embodiments taken in conjunction with the accompanying drawings, wherein:

1 is a schematic structural diagram of a two-degree-of-freedom parallel robot with a spatial branched chain structure according to Embodiment 1 of the present invention;

2 is a schematic structural diagram of a two-degree-of-freedom parallel robot with a spatial branched chain structure according to Embodiment 2 of the present invention;

Fig. 3 is the enlarged view of circled A area in Fig. 2;

FIG. 4 is an enlarged view of area B circled in FIG. 2 .

Reference number:

Two-degree-of-freedom parallel robot 100,

Fixed platform 1.

Drive device 2, active arm 21, weight reduction hole 211, drive unit 22,

Sub-branch 3, shaft rod 31, upper shaft rod 311, lower shaft rod 312, chain rod 32, sliding hole 321, anti-torsion rod 33,

Moving platform 4, center plate 41, ear plate 42, ear hole 421,

The end bracket 5, the support plate 51, the upper extension ear 52, the lower extension ear 53,

Reinforcing rod 6.

Detailed ways

The following describes in detail the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary, and are intended to explain the present invention and should not be construed as limiting the present invention.

In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "vertical", "horizontal", etc. is based on the orientation or position shown in the accompanying drawings The relationship is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

In addition, the terms "first" and "second" are only used for descriptive purposes, and should not be construed as indicating or implying relative importance or implying the number of indicated technical features. Thus, a feature delimited with "first", "second" may expressly or implicitly include at least one of that feature. In the description of the present invention, "plurality" means at least two, such as two, three, etc., unless otherwise expressly and specifically defined.

In the present invention, unless otherwise expressly specified and limited, the terms "installed", "connected", "connected", "fixed" and other terms should be understood in a broad sense, for example, it may be a fixed connection or a detachable connection , or integrated; it can be a mechanical connection or an electrical connection or can communicate with each other; it can be directly connected or indirectly connected through an intermediate medium, it can be the internal connection of two components or the interaction relationship between the two components, unless otherwise expressly qualified. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

The following describes a two-degree-of-freedom parallel robot 100 with a spatial branch structure according to an embodiment of the present invention with reference to FIGS. 1 to 4 .

A two-degree-of-freedom parallel robot 100 with a spatial branch chain structure according to an embodiment of the present invention, as shown in FIG. 1 and FIG.

Among them, two driving devices 2 are set on the fixed platform 1, and each driving device 2 includes a driving arm 21 and a driving unit 22 for driving the driving arm 21 to rotate. The driving unit 22 is set on the fixed platform 1, and the two driving devices The arms 21 lie in the same reference plane. That is to say, one end of the two active arms 21 is pivotably connected to the fixed platform 1 , the pivot axes of the two active arms 21 are parallel, and the plane formed by the two active arms 21 is called the reference plane.

The moving platform 4 has a main plane which is always perpendicular to the reference plane. It is proposed here that the moving platform 4 has a main plane, and the main plane is a virtual plane, which means that the movement of the moving platform 4 relative to the reference plane is restricted, and the moving platform 4 can only translate along the reference plane, or the axis of rotation of the moving platform 4 can only be perpendicular to the reference plane. reference surface.

As shown in FIG. 1 and FIG. 2 , a sub-branch 3 is connected between each active arm 21 and the moving platform 4 , and the sub-branch 3 is a parallelogram rod group. Each sub-chain 3 includes: two equal-length shaft rods 31 and two equal-length chain rods 32, the two shaft rods 31 are arranged in parallel, one of the two shaft rods 31 is arranged on the driving arm 21, and the other is arranged on the driving arm 21. On the moving platform 4 , both ends of the two chain rods 32 are respectively connected with the two shaft rods 31 . Here, each sub-chain 3 is a four-link structure with four connection points, the two shaft rods 31 being equal in length means that the lengths of the two shaft rods 31 between the connection points are of equal length, and the two chain rods 32 are of equal length. To be long means that the length of the two link rods 32 between the connection points is equal in length. The two shaft rods 31 and the two chain rods 32 of a sub-branch 3 form a parallelogram. It should be emphasized here that, as shown in FIG. 3 and FIG. 4 , each shaft rod 31 and each link rod 32 are spherically hinged to ensure flexibility when the shape of the parallelogram changes.

Wherein, at least one active arm 21 is hinged with an end bracket 5, and the end bracket 5 is connected with the fixed platform 1 through the reinforcing rod 6. The fixed platform 1, the active arm 21, the end bracket 5 and the reinforcing rod 6 form a parallelogram.

As shown in FIG. 1 , when the driving arm 21 is directly connected with the sub-branch 3 , one sub-branch 3 is connected to the driving arm 21 . As shown in FIG. 1 and FIG. 2 , when the main arm 21 is provided with the end bracket 5 , the sub-branch 3 is connected to the end bracket 5 to indirectly connect the main arm 21 , and two sub-branches are connected to one end bracket 5 3. Moreover, when the sub-branch 3 is connected to the main arm 21 through the end bracket 5, the shaft 31 on the end bracket 5 is not coaxial with the rotation axis of the main arm 21, and the two sub-branches 3 connected to the same end bracket 5 are opposite to each other. The reference plane is symmetrical.

Moreover, in the two sub-chains 3 connected to the same end bracket 5, the two shaft rods 31 connected to the end bracket 5 form an included angle greater than 0 degrees and less than 180 degrees, and the two shafts connected to the moving platform 4 The rod 31 forms an included angle greater than 0 degrees and less than 180 degrees. In the two sub-branches 3 on the same end bracket 5 , the distance between the two shaft rods 31 connected to the end bracket 5 is greater than the distance between the two shaft rods 31 connected to the moving platform 4 . The driving device 2 , the end support 5 and the two sub-branches 3 connected to the same end support 5 together constitute a space branch of the two-degree-of-freedom parallel robot 100 with a space branch structure.

That is to say, in the present invention, the two-degree-of-freedom parallel robot 100 may have two basic configurations, namely the configuration shown in FIG. 1 and the configuration shown in FIG. 2 .

In FIG. 1 , two driving devices 2 are connected to the fixed platform 1 , and each driving device 2 includes a driving arm 21 and a driving unit 22 for driving the driving arm 21 to rotate. The end of one of the driving arms 21 is not connected to the end bracket 5 , and the end of the other driving arm 21 is connected to an end bracket 5 . A reinforcement rod 6 is also connected between the end bracket 5 and the fixed platform 1 , and the fixed platform 1 , the active arm 21 , the end bracket 5 and the reinforcement rod 6 form a parallelogram.

Only one sub-branch 3 is connected to the active arm 21 that is not connected to the end bracket 5 , while two sub-branches 3 are connected to the end bracket 5 . Each sub-chain 3 includes: two shaft rods 31 and two chain rods 32, the two shaft rods 31 are arranged in parallel, one of the two shaft rods 31 is arranged on the driving arm 21, and the other is arranged on the moving platform 4, Both ends of the two chain rods 32 are connected with the two shaft rods 31 respectively, the two shaft rods 31 and the two chain rods 32 of a sub-branch 3 form a parallelogram, and each shaft rod 31 and each chain rod 32 form a parallelogram. There is a spherical hinge between them.

For the convenience of description here, referring to the direction shown in FIG. 1 , the moving platform 4 is located below the fixed platform 1 in FIG. 31 is the lower shaft 312 . In the following, the positions of the various components are described with reference to the directions shown in FIG. 1. Of course, FIG. 1 is only for example, and does not indicate or imply that the devices or elements referred to must have a specific orientation or be constructed in a specific orientation. and operation. Besides, in the following, for the convenience of describing the working principle of the parallel robot with reference to the accompanying drawings, the Cartesian coordinate system composed of the X-axis, the Y-axis and the Z-axis shown in FIG. 1 is also introduced. In the Cartesian coordinate system, the Z-axis direction corresponds to the up-down direction.

In the two sub-branches 3 on the same end bracket 5, an included angle greater than 0 degrees and less than 180 degrees is formed between the two upper shaft rods 311, and an included angle greater than 0 degrees and less than 180 degrees is formed between the two lower shaft rods 312, And the distance between the two sub-branches 3 on the end bracket 5 is greater than the distance on the moving platform 4 . The driving device 2 , the end support 5 and the two sub-branches 3 connected to the end support 5 together form a space branch of the two-degree-of-freedom parallel robot 100 with a space branch structure.

The basic configuration of the two-degree-of-freedom parallel robot 100 shown in FIG. 1 completely restricts the three rotational degrees of freedom of the moving platform 4 around the X-axis, the Y-axis and the Z-axis, and also completely restricts the movement of the moving platform 4 along the Y-axis. translational degrees of freedom. The two-degree-of-freedom parallel robot 100 can realize the movement degrees of freedom along the X-axis and the Z-axis, has the characteristics of compact structure, easy assembly, high control precision and large working space, and can achieve high-speed and high-acceleration characteristics.

In FIG. 2 , two driving devices 2 are connected to the fixed platform 1 , and each driving device 2 includes a driving arm 21 and a driving unit 22 for driving the driving arm 21 to rotate. The ends of the two active arms 21 are connected with end brackets 5 , and a reinforcing rod 6 is connected between each end bracket 5 and the fixed platform 1 . On both sides of the fixed platform 1, the fixed platform 1, the active arm 21, the end bracket 5 and the reinforcing rod 6 all form a parallelogram.

Two sub-branches 3 are connected to the two end brackets 5, and each sub-branches 3 includes: two shaft rods 31 and two chain rods 32, and the two shaft rods 31 are the upper shaft rod 311 and the lower shaft arranged in parallel. The rod 312, the upper shaft rod 311 is set on the end bracket 5, the lower shaft rod 312 is set on the moving platform 4, the two ends of the two chain rods 32 are respectively connected with the two shaft rods 31, and the two The shaft rod 31 and the two link rods 32 form a parallelogram, and each shaft rod 31 and each link rod 32 are spherically hinged.

In the two sub-branches 3 on the same end bracket 5, an included angle greater than 0 degrees and less than 180 degrees is formed between the two upper shaft rods 311, and an included angle greater than 0 degrees and less than 180 degrees is formed between the two lower shaft rods 312, And the distance between the two sub-branches 3 on the end bracket 5 is greater than the distance on the moving platform 4 . Each driving device 2 , the corresponding end support 5 and the two sub-branches 3 connected to the end support 5 together constitute a space branch of the two-degree-of-freedom parallel robot 100 with a space-branch structure. The two-degree-of-freedom parallel robot 100 has two steric branches.

The basic configuration of the two-degree-of-freedom parallel robot 100 shown in FIG. 2 is an enhanced version of the basic configuration of the two-degree-of-freedom parallel robot 100 shown in FIG. 1 . The two-degree-of-freedom parallel robot 100 shown in FIG. 2 can also achieve three rotational degrees of freedom around the X-axis, Y-axis and Z-axis and one translational degree of freedom along the Y-axis of the two-degree-of-freedom parallel robot 100 shown in FIG. 1 . The four-degree-of-freedom restriction scheme, that is, the two-degree-of-freedom parallel robot 100 shown in FIG. 2 can also realize the movement degrees of freedom along the X-axis and the Z-axis. Moreover, on the two-degree-of-freedom parallel robot 100 shown in FIG. 2 , the two active arms 21 are connected to the two sub-chains 3 through the end brackets 5 , forming a redundant structure can further improve the robot stiffness and the end output characteristics of the movable platform 4 .

The two-degree-of-freedom parallel robot 100 according to the embodiment of the present invention has the following advantages:

1. The arrangement on both sides of the end bracket 5, that is, the interaction of the active arm 21-reinforcing rod 6 combined rod and the double-branched combined rod, strictly limits the four degrees of freedom of the moving platform 4, and the moving platform 4 is only the remaining one. Under the two degrees of freedom, the whole machine has high rigidity, high control precision, easy assembly, performance consistency and retention.

2. The drive unit 22 is fixed on the fixed platform 1 instead of moving with the movement of the drive chain like a traditional serial robot. This arrangement is conducive to realizing the lightweight of the drive chain and improving the dynamic response performance of the drive chain. Moreover, after the drive chain is freed from the burden of the drive unit 22, energy consumption can also be reduced.

Preferably, the drive unit 22 is preferably an electric motor. Of course, in other embodiments of the present invention, a hydraulic drive mechanism may also be used to replace the electric motor. For example, when multiple two-degree-of-freedom parallel robots 100 are operated on a production line, hydraulic cylinders (through pistons, gear systems, etc.) can be used to transmit power to each active arm 21, and the cost of linkage setup can be effectively controlled.

Specifically, both ends of each link rod 32 are formed with hinge holes, the inner wall of the hinge hole is a spherical surface, both ends of the shaft rod 31 are formed as ball heads, and the ball heads at both ends of the shaft rod 31 are matched with the two link rods 32 in the hinge hole on the top to form a spherical hinge.

In some embodiments, as shown in FIG. 1 to FIG. 4 , in each sub-chain 3 , at least one anti-torsion bar 33 is connected between the two chain rods 32 , and the anti-torsion rod 33 is the same length and parallel to the shaft rod 31 . set up. The torsion bar 33 can more effectively prevent the torsion of the sub-branch chain 3 and ensure the stability of the work of the sub-branch chain 3 .

Specifically, as shown in FIG. 3 and FIG. 4 , each chain rod 32 is provided with a sliding hole 321 extending along its length direction, and the rotating shaft of the anti-torsion rod 33 passes through the sliding hole 321 . With the arrangement of the sliding hole 321, the adaptability of the sub-branch 3 at various angles can be improved, and the flexibility can be improved.

In some embodiments, as shown in FIG. 1 , the moving platform 4 includes a center plate 41 and a plurality of pairs of ear plates 42 , each pair of ear plates 42 is connected to the outer circumference of the center plate 41 , and each pair of ear plates 42 includes spaced apart devices. There are two ear plates 42 with ear holes 421 , and each shaft 31 is connected to the ear holes 421 of a pair of ear plates 42 . The moving platform 4 has a simple structure and is easy to assemble.

In some embodiments, the driving arm 21 is provided with a weight reduction hole 211 . The setting of the weight-reducing hole 211 can reduce the weight of the active arm 21 and enhance the dynamic characteristics of the robot.

In some embodiments, as shown in FIG. 3 , the end bracket 5 includes a support plate 51 , a pair of upper extension ears 52 and two pairs of lower extension ears 53 . The active arm 21 is hinged on the support plate 51 , a pair of upper extension ears 52 are provided on one side of the support plate 51 , and the reinforcing rod 6 is hinged between the pair of upper extension ears 52 . Two pairs of lower extension ears 53 are connected to the other side of the support plate 51 , and each pair of lower extension ears 53 is connected to the shaft 31 of a sub-branch 3 .

Other components of the two-degree-of-freedom parallel robot 100 with spatial branches according to the embodiment of the present invention, such as motors, etc., and operations are known to those of ordinary skill in the art, and will not be described in detail here.

In the description of this specification, description with reference to the terms "embodiment," "example," etc. means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention . In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

Although embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, The scope of the invention is defined by the claims and their equivalents.

Claims (3)

1. A two-degree-of-freedom parallel robot with a spatial branched chain structure is characterized by comprising:

fixing a platform;

each driving device comprises a driving arm and a driving unit for driving the driving arm to rotate, the driving unit is arranged on the fixed platform, and the two driving arms are positioned in the same reference plane;

the movable platform is provided with a main plane which is always vertical to the reference plane;

the sub-branched chain is connected between each driving arm and the movable platform, and each sub-branched chain comprises: the two shaft levers are arranged in parallel, one of the two shaft levers is arranged on the active arm, the other one of the two shaft levers is arranged on the movable platform, two ends of the two chain levers are respectively connected with the two shaft levers to form a parallelogram, and each shaft lever is hinged with each chain lever through a spherical surface; wherein,

a tail end support is hinged to at least one active arm, the tail end support is connected with the fixed platform through a reinforcing rod, and the fixed platform, the active arm, the tail end support and the reinforcing rod form a parallelogram; and,

when the main driving arm is directly connected with the sub-branched chain, the main driving arm is connected with one sub-branched chain;

when the main arm is provided with the tail end support, the sub-branched chains are connected to the tail end support so as to be indirectly connected with the main arm, one tail end support is connected with two sub-branched chains, and the two sub-branched chains connected to the same tail end support are symmetrical relative to the reference plane;

in addition, in the two sub-branched chains connected to the same terminal bracket, the two shaft levers connected to the terminal bracket form an included angle larger than 0 degree and smaller than 180 degrees, the two shaft levers connected to the movable platform form an included angle larger than 0 degree and smaller than 180 degrees, and the distance between the two shaft levers connected to the terminal bracket is larger than the distance between the two shaft levers connected to the movable platform;

the shaft lever on the tail end support is not coaxial with the rotating shaft of the driving arm;

the driving device, the tail end bracket and the two sub-branched chains connected to the same tail end bracket jointly form a space branched chain of the two-degree-of-freedom parallel robot with a space branched chain structure; in each sub-branched chain, at least one torsion resistant rod is connected between two chain rods, and the torsion resistant rod is equal to the shaft rod in length and is arranged in parallel; each chain rod is provided with a sliding hole extending along the length direction of the chain rod, and a rotating shaft of the torsion bar penetrates through the sliding hole.

2. The two-degree-of-freedom parallel robot having a spatial branched structure according to claim 1, wherein the movable platform includes a central plate and a plurality of pairs of ear plates, each pair of ear plates being connected to an outer circumference of the central plate, each pair of ear plates including two ear plates provided with ear holes at intervals, each of the shafts being connected to the ear holes of a pair of the ear plates.

3. The two-degree-of-freedom parallel robot having a spatial branched structure according to claim 1, wherein the end bracket comprises:

the support plate is hinged to the driving arm;

the pair of upper extending lugs is arranged on one side of the support plate, and the reinforcing rod is hinged between the pair of upper extending lugs;

and the two pairs of lower extending lugs are connected to the other side of the support plate, and each pair of lower extending lugs is connected with the shaft lever of one sub-branched chain.

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